1. Field of the Invention
This invention relates in general to apparatus and method for tuning electromechanical filters and in particular the resonators of an electromechanical filter which are intermediate between the input and output resonators.
2. Description of the Prior Art
Electromechanical filters which comprise a plurality of mechanically vibrating resonators mechanically coupled together and with an input transducer on one of the resonators and an output transducer on another resonator and with at least one resonator mounted between the input and output resonators upon which no transducer is mounted are well known.
It is also well known that the satisfactory operation of electrical mechanical filters depends upon the individual resonators being tuned with a high degree of accuracy to specific predetermined resonant frequency. The individual resonators are manufactured and selected to be approximately at the correct frequencies, but due to unavoidable production tolerances the individual resonators of a completed filter after assembly must, in many instances, be additionally adjusted to the particular required resonant frequency. For example, the physical dimensions of a resonator determines its resonant frequency and the frequency can be adjusted by sand blasting or by using laser radiation so as to remove material from the resonators and, thus, change their frequencies. There is a difficulty of determining the resonant frequency of an individual resonator, however, because they are mechanically coupled to adjacent resonators and this coupling causes the adjacent resonators to influence the indicated tuning and leads to false indications of the resonant frequency of the particular resonator.
It is known in the prior art to avoid the influence of adjacent resonators on the resonator which is to be adjusted during the adjustment process to mechanically clamp the adjacent resonators so they cannot vibrate. For this purpose, expensive mechanical pressing mechanisms are required and such presses require that the clamping faces of the resonator be as flat as possible and that they be aligned parallel to one another. Such clamps make it difficult or impossible to tune by removing material on the clamping faces. Also, frequently it is not possible to effect clamping at those points of a resonator which are subject to the greatest amount of movement, since generally electrostrictive ceramics are used as transducers at the driving and output points of the resonator and if the clamping pressure is applied at those points, such clamping force would destroy the ceramic transducers.
German Pat. No. 1,245,507 suggests replacing mechanical clamping by so-called electrical clamping which is achieved by establishing the condition that those resonators which directly adjoin the resonator to be tuned operate as compound oscillators with their electrostrictive transducers and during tuning the transducers are connected in parallel with a coil having a value of an inductance which is such that together with the static capacitance of the electrostrictive transducers a parallel resonant circuit is formed which is tuned to the theoretical frequency of the resonator to be adjusted including the coupling element. In this manner, the resonators which are directly adjacent to the resonator which is to be tuned are heavily damped so that their effects on the resonator to be tuned are negligible during the adjustment process. The oscillation excitation and measurement of the oscillating frequency of the resonator to be tuned is however effected by the electrostrictive transducers which must be connected or coupled to the resonator to be tuned.
However, the adjustment of resonators which contain no transducers requires a different oscillation excitation and measurement to determine the resonant frequency. In this case, the resonator can be caused to oscillate with a drive device, for example, a driving coil with the aid of the magnetostriction effect. The resonant frequency of the resonator which is to be measured can be detected by a microphone mounted in the direction vicinity of the resonator and such signal is generally fed to a feedback amplifier which forms a measuring circuit together with the drive device and the microphone. This allows the desired resonant frequency to be measured. However, the self-resonant frequency of the resonator to be tuned is displaced in relation to the self-resonant frequency which occurs in the unmagnetized state. This frequency shift occurs as a result of the dependency of the module of elasticity E of the resonator material upon the degree of bias magnetization and in the case of some resonator materials using powerful bias magnetization values for the relative frequency change .DELTA..sub.f /f can be up to 2.multidot.10.sup.-4. A disadvantage of such oscillation measurement also results from the heavy microphone wear and tear which occurs during the sand blasting operation. Additionally, ultrasonic waves that occur during the sand blasting adjustment can cause erroneous measurements. Another disadvantage is that non-uniform magnetization of the individual resonators occurs with this technique due to unavoidable variations in the materials used in the resonators which makes it very difficult to automate this known process.